1. Field of the Invention
The present invention relates to optical devices, such as wavelength routers and optical multiplexers, used in light-based telecommunications and computer networks.
2. Description of the Related Art
The demand for higher and higher communication and computer data rates implies a constant need for newer and better technologies to support that demand. One such technology area is fiber-optic communications, in which data is transmitted as light energy over optical fibers. To increase data rates, more than one data channel can be placed on a single fiber link. For example, in wavelength division multiplexing (WDM), the different channels are differentiated by wavelength or color. Such techniques require special components to combine and/or separate the different channels for transmission, switching, and/or receiving.
A wavelength router (also known as a waveguide grating router, an arrayed waveguide grating, or a phased array) is an optical device that can be used to combine and/or separate light energy of different wavelengths. A wavelength router selectively routes light of a particular wavelength from an input port to an output port. When used to route light of differing wavelengths from more than one input port and/or to more than one output port, a wavelength router can be used to operate as an optical multiplexer and/or demultiplexer that combines and/or separates light energy of different wavelengths.
FIG. 1 shows a schematic diagram of a conventional wavelength router 100 implemented as an integrated device formed on a suitable substrate 102 (e.g., silicon or silica). Router 100 has a plurality of input waveguides 106 adapted to receive light from one or more incoming optical fibers that can be connected to one or more of the input ports 104. Router 100 also has a plurality of output waveguides 114 adapted to transmit light to one or more outgoing optical fibers that can be connected to one or more of the output ports 116. Between the input and output waveguides are two free spaces 108 and 112 separated by a set of waveguides that form the arms 110 of the router.
In operation, light received at one of the input ports 104 is transmitted along the corresponding input waveguide 106 to free space 108. Light entering free space 108 gets radiated for receipt by--and transmission along--each of the router arms 110 towards free space 112. Light entering free space 112 gets radiated towards the output waveguides 114.
Wavelength router 100 is preferably designed such that all of the optical distances from a particular location at the input side of free space 108 (i.e., where one particular of the input waveguides 106 meets free space 108) along each router arm 110 to a particular location on the output side of free space 112 (i.e., where one particular of the output waveguides 114 meets free space 112) differ by an integer multiple of a particular wavelength for the different router arms. As such, light of that particular wavelength entering free space 108 from that particular input waveguide 106 will be focused on the output side of free space 112 at that particular output waveguide 114. That is, light of that particular wavelength will constructively interfere (i.e., add in phase) at that particular output waveguide location, and substantially destructively interfere at all other output waveguide locations. Moreover, light of most other wavelengths will not, in general, be focused (i.e., will effectively destructively interfere) at that particular output waveguide location. As such, wavelength router 100 can be used as an optical passband filter.
Furthermore, to the extent that wavelength router 100 can be designed to focus light having different wavelengths at different output waveguide locations on the output side of free space 112, router 100 can operate as a one-to-many optical multiplexer that can receive light of different wavelengths from a single incoming optical fiber and selectively transmit those different frequencies to different output ports for propagation along different outgoing optical fibers. Similarly, router 100 can be further designed to operate as a many-to-one optical demultiplexer that receives different wavelength light from different incoming optical fibers for transmission to a single outgoing optical fiber, or as a many-to-many optical multiplexer that receives different wavelength light from different incoming optical fibers for transmission to different outgoing optical fibers. Moreover, router 100 may be a symmetric optical device that can be operated in either direction (i.e., either from left to right or from right to left in FIG. 1). Typically, the router is realized using silica waveguides deposited on a thick substrate of quartz or silicon.
Since optical communications systems are typically deployed in the real world, they must operate effectively over a relatively wide range of temperatures. As such, it is desirable to design optical devices such as wavelength routers whose operating characteristics do not change significantly over that temperature range. Due to thermal expansion and index changes, changes in temperature can result in changes in the optical path lengths of the router arms. Since the router arms have different lengths, the optical path lengths of the different router arms will change by different amounts. This can adversely affect the ability of a wavelength router to operate effectively as temperature changes. Previous attempts to design athermal wavelength routers, that is, routers that are relatively independent to temperature fluctuations, have ultimately proven unsatisfactory. Consequently, the temperatures of wavelength routers are controlled by expensive heaters. Typical heaters require a few watts of power. This power requirement limits the use of wavelength routers in outside plant (non-central office) locations and in remote locales (such as in the ocean).